Ceramic arc tube assembly

ABSTRACT

A ceramic arc tube assembly and a method for making a ceramic arc tube are described which simplify the manufacture of ceramic arc tubes by reducing the number of handling and heat treatments required to assemble arc tubes prior to the final sintering operation. In particular, the invention uses transient assembly buttons during intermediate assembly steps which are removed prior to the final sintering operation.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/271,153, filed Feb. 23, 2001.

TECHNICAL FIELD

This invention relates to ceramic arc tubes and more particularly toceramic arc tube assemblies and methods of making such assemblies.

BACKGROUND OF THE INVENTION

Over the years ceramic arc tubes composed of materials such aspolycrystalline alumina have been used to contain the discharges ofhigh-pressure sodium lamps. This has resulted in the development of anumber of arc tube configurations for use in sodium lamps. For example,U.S. Pat. No. 4,766,347 describes a three-piece arc tube configurationwherein the arc tube comprises a ceramic body with tubular closuremembers. The closure members which receive the electrodes are sealeddirectly in the ends of the ceramic body. In another example, U.S. Pat.No. 5,426,343 describes a three-piece arc tube configuration whereinend-sealing buttons are used which have an extending electrode-receivingmember integral therewith.

More recently, ceramic arc tubes have been employed in metal halidelamps. For example, U.S. Pat. No. 5,424,609 describes a five-piececeramic arc tube configuration for a metal halide lamp. The five-piecearc tube includes a cylindrical body, a pair of end-sealing buttons, anda pair of capillary tubes sealed to the buttons. The manufacture ofthese ceramic arc tubes requires extrusion or pressing of the individualcomponents, as well as multiple assembly and heat treatment steps. Thesemultiple steps result in increased handling which increases themanufacturing cost.

SUMMARY OF THE INVENTION

It is an object of the invention to obviate the disadvantages of theprior art.

It is another object of the invention to provide a ceramic arc tubeassembly which facilitates and simplifies the manufacturing of ceramicarc tubes.

It is another object of the invention to provide a method for makingceramic arc tubes which reduces the number of handling and firing stepsin the manufacturing process.

In accordance with one object the invention, there is provided a ceramicarc tube assembly comprising a hollow body having at least one open endcontaining an end cap, the end cap having a capillary tube and atransient assembly button, the capillary tube extending outwardly fromthe hollow body and having a length inserted into the open end andforming a seal with the hollow body, the transient assembly button beingfixed around the capillary tube and contacting an edge of the open end,the interaction between transient assembly button and the edge of theopen end determining the length of the capillary tube inserted into theopen end, and the transient assembly button being capable of removalfrom the capillary tube without causing damage to the arc tube assembly.

In accordance with another object of the invention, there is provided amethod of making a ceramic arc tube comprising the steps of:

(a) fixing a transient assembly button around a capillary tube to forman end cap;

(b) inserting the end cap into an open end of a hollow arc tube bodyuntil the transient assembly button contacts an edge of the open end;

(c) heating the assembly to form a mechanical seal between the capillarytube and the open end of the hollow body;

(d) removing the transient assembly button without damaging theassembly; and

(e) sintering the assembly to form the ceramic arc tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an end cap for a three-piece ceramicarc tube assembly.

FIG. 2 is a cross-sectional view of a three-piece ceramic arc tubeassembly.

FIG. 3 is a cross-sectional view of a completed ceramic three-piece arctube.

FIG. 4 is a cross-sectional view of an end cap for a five-piece ceramicarc tube assembly.

FIG. 5 is a cross-sectional view of a five-piece ceramic arc tubeassembly.

FIG. 6 is a cross-sectional view of a completed ceramic five-piece arctube.

FIG. 7 is a cross-sectional view of another embodiment of a five-piececeramic arc tube assembly.

FIG. 8 is an illustration of a grooved transient assembly button.

FIG. 9 is an illustration of a notched transient assembly button.

DESCRIPTION OF PREFERRED EMBODIMENTS

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawings.

The ceramic arc tube assembly and method of this invention simplify themanufacture of ceramic arc tubes by reducing the number of handling andheat treatments required to assemble the arc tube prior to the finalsintering operation. In one embodiment, the ceramic arc tube assemblycomprises a hollow arc tube body having at least one open end containingan end cap. The end cap comprises a transient assembly button and acapillary tube for receiving an electrode. The transient assembly buttonis fixed around the capillary tube by an initial heat treatment whichcauses the button to shrink and compress against the capillary tube.When the components are being assembled, the transient assembly buttoninteracts with the edge of the open end to limit the penetration of theend cap into the arc tube body. This interaction fixes the length of thecapillary tube inserted into the arc tube body. After the components areassembled, the end cap is then joined to the arc tube body in a secondheat treatment step which mechanically seals the end cap to the arc tubebody. The completed arc tube is then made by removing the transientassembly button and firing the assembled arc tube in a final sinteringoperation.

In another embodiment, the capillary tube has a radially-extendingsealing member which is inserted into the open end of the arc tube bodyand sealed therein. The sealing member may comprise a separate partwhich is joined to the capillary tube along with the transient assemblybutton in the initial heat treatment or it may be formed as an integralpart thereof. Preferably, the transient assembly button has the form ofan annular disc whereby the capillary tube is inserted into the centeraperture. However, other shapes may also be equally effective providedthat the transient assembly button can be removed prior to the finalsintering operation without damaging the arc tube assembly.

When the both ends of the arc tube require end caps to be sealedtherein, the use of the transient assembly buttons is particularlyadvantageous as it allows both ends of the arc tube assembly to besealed simultaneously in a vertical orientation. This simplifies theassembly process by eliminating the need to seal each end of the bodyportion to an end cap in two sequential heat treatment operations at twodifferent temperatures.

The ceramic arc tube assembly of the present invention is preferablyformed from polycrystalline alumina containing minor amounts of magnesiaand, in some instances, yttria and zirconia. Such a material is shown inU.S. Pat. No. 5,682,082. Other minor constituents may also be includedas is known in the art. In a preferred embodiment, the end caps of thearc tube assemblies (capillaries, transient assembly buttons, sealingmembers) are made of Baikowski grade CR-6 alumina powder containing 0.05weight percent magnesia and the body portion of either Baikowski gradeCR-6 or Baikowski grade CR-30 powder containing 0.05 weight percentmagnesia. The components of the ceramic arc tube assembly may be formedby a number of conventional methods, e.g., extrusion, pressing, orinjection molding.

The relative positions of the components in the arc tube assembly arefixed in a heat treatment step at or below about 1350° C. The heattreatment causes the components to densify and shrink resulting incompressive forces which fix the positions of the components. Because ofthe relatively low temperature of the heat treatment step, there is nocross diffusion or grain growth between the components to bond themtogether chemically. In addition, at temperatures below 1350° C., thetransient assembly buttons retain considerable porosity which limittheir strength. This allows the transient assembly buttons to be easilysnapped off of the capillary tubes without damaging the arc tubeassembly.

FIGS. 1-3 illustrate the various stages of manufacture for a three-piececeramic arc tube using the invention described herein. In FIG. 1, an endcap 3 comprised of capillary tube 2 and transient assembly button 11 isformed by placing capillary tube 2 through a hole in perforated ceramicplate 5 (shown in dotted line) of controlled thickness L. The perforatedceramic plate 5 rests on solid ceramic plate 10 (also shown in dottedline) and prevents the capillary tube from passing completely throughthe perforated plate. Transient assembly button 11 is placed on thecapillary tube 2 and is supported by the perforated ceramic plate 5. Theplates and the supported end cap components are then placed in a furnaceand heated in air at from about 1200° C. to about 1250° C. This initialheat treatment causes transient assembly button 11 to shrink and becomefixed around the capillary tube. The thickness L of the perforatedceramic plate corresponds to the length of capillary tube 2 which willbe inserted into the arc tube body.

In FIG. 2, an arc tube assembly is made by placing the capillary 2 a ofan end cap 3 a through a hole in perforated ceramic plate 5 (shown indotted line) with the protruding end of the capillary 2 a extendingbelow the plate and transient assembly button 11 a supported on theplate. A hollow elliptical body portion 4 having open ends 7 a, 7 b isplaced over the capillary portion of the end cap 3 a. A second end cap 3b is inserted into the opposite end 7 b of the hollow body. Theperiphery 17 a, 17 b of the transient assembly buttons 11 a, 11 b ofeach end cap 3 a, 3 b must be greater than the inside diameter of theopen ends 7 a, 7 b just prior to assembly. The length of capillary tubes2 a, 2 b inserted into the open ends 7 a, 7 b of the hollow body isdetermined by the interaction between the edges 13 a, 13 b of the openends and the transient assembly buttons 11 a, 11 b. It is preferred thatafter assembly the periphery 17 a, 17 b of the transient assemblybuttons 11 a, 11 b be greater than the periphery 19 a, 19 b of the openends 7 a, 7 b to facilitate removal of the transient assembly buttons.Preferably, the transient assembly buttons extend about 1 mm beyond theperiphery of the open ends. The assembled components are then heattreated in air at from about 1250° C. to about 1350° C. during which thebody portion densities sufficiently to shrink onto the capillary tubesforming mechanical seals 9 a, 9 b and a completed arc tube assembly.

The transient assembly buttons are then removed from the capillary tubeswithout damaging the arc tube assembly. The assembly can be fired eitherhorizontally, or, in the preferred method, vertically in a finalsintering operation at a temperature above about 1800° C. in ahydrogen-containing atmosphere. Preferably, the final sintering isperformed at about 1880° C. for about 180 minutes in 100% hydrogen. Thefinished arc tube is shown in FIG. 3. The final sintering operationpromotes grain growth and interdiffusion at the existing mechanicalseals in the arc tube assembly in combination with further shrinkage.These actions cause the formation of hermetic seals 9 a′, 9 b′ in thesintered assembly which are both vacuum and gas tight.

FIGS. 4-6 illustrate the various stages of manufacture for a five-piececeramic arc tube using the invention described herein. In FIG. 4, theend cap 23 is shown comprised of a capillary tube 2, sealing member 25and transient assembly button 11. The periphery 17 of transient assemblybutton 11 extends beyond the periphery 21 of sealing button 25. The endcap components are assembled on a ceramic plate (not shown) and heattreated in air at from about 1200° C. to about 1250° C. During thistreatment, sealing member 25 and transient assembly button 11 densifysufficiently to shrink onto capillary 2 applying a compressive forcewhich fixes the relative positions of the components.

The arc tube assembly shown in FIG. 5 is made by inserting the capillary2 a of end cap 23 a through a hole in perforated ceramic plate 5 (shownin dotted line) with the protruding end of the capillary 2 a-extendingbelow the plate and transient assembly button 11 a supported on theplate. A cylindrical hollow arc tube body 24 having opposed open ends 27a, 27 b is placed over the sealing member 25 a of end cap 23 a.

A second end cap 23 b is inserted into the opposite open end 27 b of thecylindrical body portion. The periphery 17 a, 17 b of transient assemblybutton 11 a, 11 b must be greater than the inside diameter of the openends 27 a, 27 b of the cylindrical body just before assembly. Theinteraction between the transient assembly buttons 11 a, 11 b and theedges 33 a, 33 b of open ends 27 a, 27 b determines the insertion lengthof the capillary tubes 2 a, 2 b. It is preferred that after assembly,the periphery 17 a, 17 b of the transient assembly buttons 11 a, 11 bextend beyond the outside diameter of the open ends 27 a, 27 b tofacilitate removal of the transient assembly buttons.

The arc tube assembly is then heat treated in air at from about 1250° C.to about 1350° C. to cause the cylindrical body to shrink onto thesealing members 25 a, 25 b to form mechanical seals 29 a, 29 b. Thetransient assembly buttons are then removed from the capillary tubes andthe assembly is subjected to the final sintering to form a finished arctube as shown in FIG. 6.

Another embodiment of a five-piece arc tube assembly of this inventionis shown in FIG. 7, this five-piece arc tube assembly is similar to theone shown in FIG. 5 except that the bottom end cap 23 a is not made witha transient sealing button. The end cap 23 b which utilizes transientassembly button 11 is inserted only into the upper open end 27 b of thehollow body 24. The insertion length of capillary 2 b is determined bythe interaction between the transient assembly button 11 and edge 33 bwhereas the insertion length of capillary 2 a is determined by theinteraction between the surface of perforated plate 5 (shown in dottedline) and edge 33 a. After a heat treatment to mechanically seal sealingmembers 25 a, 25 b in open ends 27 a, 27 b, the single transientassembly button is snapped off the upper capillary tube 2 b to form acompleted assembly.

In another embodiment, capillary tubes 2 may be subjected to a thermalpre-treatment prior to being joined with transient assembly buttons 11or sealing members 25. The thermal pretreatment densities the capillarytubes causing them to shrink so that they may fit through openings inthe transient assembly buttons or sealing members. The pretreatment maybe performed in air at a temperature from about 1250° C. to about 1350°C.

To better facilitate removal of the transient assembly buttons, it isoften desirable to incorporate a groove or notch in the buttons toreduce their strength further. The groove in the transient assemblybutton is typically made on one side to a depth of from about 50 percentto about 75 percent of the button thickness. The groove can be producedby a feature in the button die pressing tooling or cut by an abrasivesaw. A transient assembly button comprising a grooved annular disc isshown in FIG. 8. The groove 58 is cut along a diameter of the button 51.When using grooved transient assembly buttons, it is preferred thatduring the end cap assembly the groove be oriented so that it will facethe open end of the arc tube body when the end cap is inserted. Thisorientation allows the groove to be under maximum tensile stress duringremoval.

A second method to reduce the strength of the transient assembly buttonis to notch the button. In a preferred embodiment shown in FIG. 9, thenotch 68 extends half-way through the button 61. In this case theorientation of the transient assembly button on the end cap is notimportant.

For thicker transient assembly buttons, grooves and notches can beutilized in various combinations to achieve an improved ease of removal.

While there has been shown and described what are at the presentconsidered the preferred embodiments of the invention, it will beobvious to those skilled in the art that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

We claim:
 1. A ceramic arc tube assembly comprising: a hollow bodyhaving at least one open end containing an end cap, the end cap having acapillary tube and a transient assembly button, the capillary tubeextending outwardly from the hollow body and having a length insertedinto the open end and forming a seal with the hollow body, the transientassembly button being fixed around the capillary tube and contacting anedge of the open end, the interaction between transient assembly buttonand the edge of the open end determining the length of the capillarytube inserted into the open end; and the transient assembly button beingcapable of removal from the capillary tube without causing damage to thearc tube assembly.
 2. The ceramic arc tube assembly of claim 1 whereinthe periphery of the transient assembly button extends beyond theperiphery of the open end.
 3. The ceramic arc tube assembly of claim 1wherein the transient assembly button is grooved to facilitate removal.4. The ceramic arc tube assembly of claim 3 wherein the groove faces theopen end.
 5. The ceramic arc tube assembly of claim 1 wherein thetransient assembly button is notched to facilitate removal.
 6. Theceramic arc tube assembly of claim 1 wherein the transient assemblybutton is an annular disc.
 7. The ceramic arc tube assembly of claim 6wherein a surface of the transient assembly button is grooved along adiameter.
 8. The ceramic arc tube assembly of claim 7 wherein thegrooved surface of the transient assembly button faces the open end. 9.The ceramic arc tube assembly of claim 7 wherein the depth of the grooveis from about 50 percent to about 75 percent of the thickness of thetransient assembly button.
 10. The ceramic arc tube assembly of claim 6wherein the transient assembly button is notched.
 11. The ceramic arctube assembly of claim 6 wherein the diameter of the transient assemblybutton is greater than an outer diameter of the open end.
 12. Theceramic arc tube assembly of claim 1 wherein the length of the capillarytube inserted into the open end has a radially extending sealing memberwhich forms the seal with the hollow body.
 13. The ceramic arc tubeassembly of claim 1 wherein hollow body has two opposed open ends eachcontaining an end cap.
 14. A ceramic arc tube assembly comprising: ahollow cylindrical body having at least one open end containing an endcap, the end cap having a capillary tube and a transient assemblybutton, the capillary tube extending outwardly from the hollow body andhaving a length inserted into the open end, the length inserted into theopen end having a radially extending sealing member forming a seal withthe hollow body, a transient assembly button being fixed around thecapillary tube and contacting an edge of the open end, the interactionbetween transient assembly button and the edge of the open enddetermining the length of the capillary tube inserted into the open end;and the transient assembly button being capable of removal from thecapillary tube without causing damage to the arc tube assembly.
 15. Theceramic arc tube assembly of claim 14 wherein the transient assemblybutton is an annular disc.
 16. The ceramic arc tube assembly of claim 15wherein a surface of the transient assembly button is grooved along adiameter.
 17. The ceramic arc tube assembly of claim 16 wherein thegrooved surface of the transient assembly button faces the open end. 18.The ceramic arc tube assembly of claim 16 wherein the depth of thegroove is from about 50 percent to about 75 percent of the thickness ofthe transient assembly button.
 19. The ceramic arc tube assembly ofclaim 15 wherein the diameter of the transient assembly button isgreater than an outer diameter of the open end.
 20. The ceramic arc tubeassembly of claim 14 wherein the transient assembly button is notched.21. The ceramic arc tube assembly of claim 14 wherein hollow body hastwo opposed open ends each containing an end cap.
 22. The ceramic arctube of claim 14 wherein the sealing member is formed as an integralpart of the capillary.